Hi my sister now does Iron man triathlons, a 2.4 mile (3.8 km) swim in a 112 mile (180 km) bike ride, and a marathon (26.2 mile, 42 km).

She initially started doing triathlons about 8 years ago, her first triathlon happened after a friend encouraged her to take part in a up and coming event in just seven weeks time!! before that she was in her own admission ‘bit of a couch potato’. She told me that night she nearly died just running round the block, the next day she nearly drowned in her local swimming pool, luckily she was fine when it came to bikes. My sis is now highly ranked in the world of Triathlons, now even in her forties she’s optimistic about her chances in cycling in London 2012.
So seven weeks of stamina a lot of hard graft, some form of coaching, should do.
Ralph